1. Field of the Invention
This invention relates to a fuel reburn process and apparatus for reduction of NO.sub.x emissions resulting from the combustion of solid carbonaceous materials, such as coal, in boilers, fluidized bed combustors, and similar combustion devices. In addition to reducing NO.sub.x emissions from boilers and similar devices, the fuel reburn process and apparatus of this invention also maintain CO emissions at environmentally acceptable levels without the addition of air or oxygen downstream of the fuel reburn zone.
2. Description of Prior Art
The utilization of coal for power generation results in emission of nitric oxides (NO.sub.x) which are formed primarily as a result of oxidation of the nitrogen inherent in the coal and oxidation of molecular nitrogen present in the combustion air. Nitric oxides released in the atmosphere contribute to acid rain, accelerate the photochemical reactions responsible for smog, and result in increased ground level ozone concentrations. The emission of nitric oxides from existing coal burning power plants is governed by Title I and Title IV of the Clean Air Act Amendments of 1990. Title IV limits the allowable NO.sub.x emissions to 0.45 pounds per MBtu for tangential fired boilers and 0.50 pounds per MBtu (approximately 375 ppm) for wall fired boilers. NO.sub.x emission limits at 0.20 pounds per MBtu have been proposed in the ozone non-attainment areas under Title I, and have been targeted for decade end implementation under the Ozone Transport Region Memorandum of Understanding.
One technique for reducing the amount of NO.sub.x produced in a combustion process and, thus, emitted by said combustion process, is the use of fuel-rich reburn. Such a reburn process is taught, for example, by U.S. Pat. No. 5,020,456; U.S. Pat. No. 5,105,747; U.S. Pat. No. 5,307,746; and U.S. Pat. No. 5,205,227. Each of these U.S. patents teaches a process and apparatus for emissions reduction from waste incineration in which a combustible material is burned in a primary combustion zone in the lower region of a combustion chamber, and an oxygen deficient secondary combustion zone for mixing with the combustion products from the primary combustion zone is formed by the injection of fuel, a calcined sorbent, recirculated flue gases, and, in some instances, a carrier fluid injected into the combustion chamber above the primary combustion zone. In each case, however, overfire air is injected into the combustion chamber above the oxygen deficient secondary combustion zone to ensure complete combustion of any remaining combustible materials in the combustion products from the oxygen deficient secondary combustion zone. The use of overfire air is necessary to ensure that no unburned hydrocarbons are exhausted by the combustion process. The requirement for overfire air adds significant capital and operating costs.